Recent years, many types of image displays, such as a liquid crystal display, a PDP (plasma display panel) and an organic EL (electroluminescence) display, have been developed. As for these image displays, in parallel with an enlargement in size of a screen, a reduction in thickness of the entire display is also being promoted.
As a component constituting an image display, there is an optical laminate with a polarizing film, for example, which is disposed on each of opposite sides of a liquid crystal cell in a liquid crystal display. Generally, the optical laminate comprises: a polarizing film obtained by causing a dichroic material to be absorbed in a layer of a PVA-based resin coatingly formed on a substrate (i.e., by impregnating, with a dichroic material, a layer of a PVA-based resin coatingly formed on a substrate) to thereby dye the layer, and subjecting the dyed layer to uniaxial stretching together with the substrate; and a protective layer disposed on each of opposite sides of the polarizing layer. A polarizing film obtained by subjecting a PVA-based film to uniaxial stretching typically has a thickness of several ten μm.
In order to attain the thickness reduction of the entire image display, it is studied to reduce a thickness of the optical laminate, particularly, a thickness of the polarizing film therein, to form a thinned polarizer. For example, a method of forming such a thinned polarizer includes a technique of preliminarily subjecting a laminate of a substrate and a PVA-based resin layer to in-air stretching at an elevated temperature, and then subjecting the layer to in-water stretching in a boric acid aqueous solution (see the following Patent Document 1). This technique is capable of stretching a laminate of a substrate and a PVA-based resin layer at a higher stretching ratio than before, and thereby forming a thinned polarizing film.
However, in the case where it is attempted to subject a laminate of a substrate and a PVA-based resin layer to stretching at a high stretching ratio in order to obtain a thinned polarizing film, if adhesion force between the substrate and the PVA-based resin layer is insufficiently low, the PVA-based resin layer is likely to be undesirably peeled off from the substrate and wound around a roll, during the stretching. Moreover, when the laminate is subjected to in-water stretching in a boric acid aqueous solution as mentioned above, breaking of the PVA-based resin layer is likely to occur due to foreign substances and gas bubbles in the layer, causing the PVA-based resin layer to be completely peeled off from the substrate. In addition, during conveyance of the formed thinned polarizer, the PVA-based resin layer is also likely to be peeled off from the substrate. The peel-off of the PVA-based resin layer from the substrate has been a factor causing deterioration in production yield of thinned polarizing films, optical laminates each comprising the thinned polarizing film, or image displays each using the optical laminate.
Therefore, there is a need for enhancing the adhesion force between the substrate and the PVA-based resin layer to prevent the PVA-based resin layer from being peeled off from the substrate during the in-air stretching, the in-water stretching, or the conveyance.
As a technique for enhancing the adhesion force between the substrate and the PVA-based resin layer, it is conceivable to subject the substrate to a corona treatment or the like before forming the PVA-based resin layer thereon, for example. As a technique to be performed in combination with or as an alternative to the above technique, it is conceivable to form an easy-adhesion layer, also called “primer coating”, on the substrate (see the following Patent Document 2).
When these techniques are applied to a polarizing film formed at a conventional stretching ratio or with a conventional thickness, it is possible to enhance adhesion between a thermoplastic resin substrate and a PVA-based resin layer to some extent. However, when they are applied to a thinned polarizing film, the reality is that a sufficient result is not always obtained. Moreover, in the technique of forming an easy-adhesion layer on the substrate and then providing the PVA-based resin layer on the easy-adhesion layer, an appearance problem, such as lines due to repellence against a PVA-based resin solution during coating of the PVA-based resin layer, is observed.